Avila said Leon was the best candidate for the job, citing the mayor's leadership during the economic recession.
"During the last few years, our region has seen some of the most devastating economic times since the great depression," Avila said.
"Paul has led the city through those times by balancing every budget without raising taxes, adding thousands of jobs, putting more officers on our streets and fighting for our schools."
Leon and Avila are among six candidates seeking to succeed Gloria Negrete-McLeod in Senate District 32.
Democratic candidates also include Assemblywoman Norma Torres of Ontario, San Bernardino County Treasurer/Tax Collector Larry Walker, and Rialto Unified School District board member Joanne Gilbert. Republican Kenny Coble, a Pomona planning commissioner, is also a candidate.
The primary is set for March 12.
"His support is indicative of the bipartisan support this campaign has received, and I look forward to representing the entire community in Sacramento," Leon said.
Avila is the second Democrat to back the Republican mayor in the Senate race. Last month, Kareem Ahmed, who has been a major donor to the Democratic Party, announced his support for Leon.
But Avila's move was a big departure from earlier last week, when the newly elected councilman voiced his displeasure for candidate endorsements during a council meeting.
At that meeting, Avila specifically spoke about his belief that newspapers should not be endorsing candidates.
"When it comes to endorsements, they are out of line," he said.
The endorsement ultimately came from that same council meeting. Avila told the mayor he was rooting for him in the race, Leon said.
"I couldn't appreciate him any more," he said.
Reach Liset via email, call her at 909-483-8556, or find her on Twitter @DBOntarioNow.